It long has been recognized by plant scientists that soils encountered in many parts of the world are considered to be saline in nature since they include a sufficiently high concentration of water-soluble inorganic salts of a substantially neutral pH to impair the normal functioning of many plants.
Representative inorganic salts which when present in excess can interfere with one or more plant functions are sodium sulfate (Glauber's salt), magnesium sulfate (Epsom salt), calcium sulfate (gypsum), sodium chloride, magnesium chloride, calcium chloride, etc. When such salts are present in a concentration level above an observable threshold level, it is found that a non-halophytic plant when grown in the same is impaired, with such impairment becoming more severe as the inorganic salt concentration increases. A plurality of these salts in injurious concentrations commonly are simultaneously encountered at a given location. Such salts often impede one or more of the following: germination, emergence, normal plant growth, and seed set following emergence. Additionally, plants affected by soil salinity often have an undesirable bluish green appearance.
It has been observed that harmful soil salinity can be encountered at locations which are far removed from present day oceans. For instance, soil salinity is frequently a problem in the Great Plains area of the United States and Canada, and in many arid and semi-arid areas where crop irrigation is a common agricultural practice. In some areas a solid crust of salts may be observed on the soil surface where it is deposited as the salts move upward by capillary action and the water evaporates. In other areas streaks of salt may be observed in the soil below the soil surface. Alternatively, the deleterious salt concentration may be dispersed in a non-visible manner as largely dissolved ions throughout the soil.
The salts may have been imparted to the soil by the weathering of rocks or minerals, or during ancient times the salts may have been introduced into the soil by the evaporation of seas or by glacier deposits. Modern agricultural practices such as irrigation may also impart increasing salt concentrations when the available irrigation water evaporates and leaves previously dissolved salts behind unless such salts are somehow flushed into the ground water and away from the plant roots.
It commonly is believed that excessive concentrations of water-soluble inorganic salts in the soil work to deprive non-halophytic plants of the water and nutrients which they need. The dissolved salts in the soil can serve to increase the osmotic pressure of the solution of the same in the soil with this solution being more concentrated than the sap in the plant roots. This accordingly tends to decrease the rate at which water from the soil will enter the roots. If the solution in the soil becomes too saturated with dissolved salts, the water may actually be withdrawn from the plant roots. Thus the plants slowly starve though the supply of water and dissolved nutrients may be more than ample. Also, elements such as sodium are known to be toxic to plants when they are taken up by the plants. For additional information see reports of Saskatchewan Agriculture, Plant Industry Branch, entitled (1) "Understanding Salt-Affected Soils" by H. M. Holm and J. L. Henry, March, 1982, and (2) "Soil Salinity--A Study in Crop Tolerance and Cropping Practices" by H. M. Holm, March, 1983.
The need to develop plants which are resistant to sodium chloride has received considerable recognition by plant scientists in the past. While some plants are known which are considered to be halophytes (i.e. salt growing plants) and to grow reasonably well in the presence of saline soil, these plants often are weed species of little economic importance such as the Russian thistle, kochia, wild barley, goosefoot, samphire, desert salt grass (Distichlis stricta), greasewood (Sorobatus vermiculatus), etc. Accordingly, readily available salt tolerant crop germ plasm commonly has not been conveniently available to improve important plants, such as flax, etc. Additionally, conventional plant breeding techniques would be expected to be complex and unusually time consuming. Another complicating factor has been the marked tendency of plants to physiologically adapt to the presence of salts to some degree following exposure to the same, but to be unable to transfer such salt tolerance to progency over a number of successive generations. These epigenetic effects can lead to spurious selections which are of no real value.
When plant cells are placed in vitro, it has been recognized that spontaneous chromosomal aberrations may occur as reported in the 1981 review article by M. J. Constantin, "Chromosome Instability in Cell and Tissue Cultures and Regenerated Plants", Envir. and Exper. Bot. 21, 359-368 (1981). Presumably these changes can affect any part of a genome and are the basis of somoclonal variation--a genetically stable phenotypic change observable at the level of the whole plant. See also the article by P. J. Larkin and W. R. Scowcroft "Somoclonal Variation--A Novel Source of Variability From Cell Cultures for Plant Improvement", Theor. Appl. Genet. 60, 197-214 (1981).
In vitro tissue culture has been pursued in the past as a possible means to develop plants which are more tolerant to sodium chloride or to sea water. Representative articles which discuss this tissue culture approach are as follows:
Nabors, M. W., Daniels, A., Nadolny, L. and Brown, C., 1975. Sodium Chloride Tolerant Lines of Tobacco Cells. Plant Sci. Lett. 4; 155-159. PA0 Dix, P. J. and Street, H. E., 1975. Sodium Chloride-Resistant Cultured Cell Lines From Nicotiana sylvestris and Capsicum annuum., Plant Sci. Lett. 5; 231-237. PA0 Epstein, E., Norlyn, J. D., Russ, D. W., Kingsbury, R. W., Kelley, D. B. Cunningham, G. A. and Wrona, A. F., 1980. Saline Culture of Crops: A Genetic Approach. Science 210: 399-404. PA0 Nabors, M. W., Gibbs, S. E., Berstein, C. S. and Meis, M. E., 1980. NaCl-Tolerant Tobacco Plants From Cultured Cells. Z. Pflanzenphysiol. 97; 13-17. PA0 Orton, T. J., 1980. Comparison of Salt Tolerance Between Hordeum vulgare and H. jubatum in Whole Plants and Callus Cultures. Z. Pflanzenphysiol. 98; 105-118. PA0 Department of Botany and Plant Pathology, Colorado State University, Fort Collins, Colo., Oct. 1, 1982, Progress Report Tissue Culture for Crops. PA0 Henke, R. R., 1981. Selection of Biochemical Mutants in Plant Cell Cultures: Some Considerations., Envir. and Exp Bot , 21, No. 3/4; 347-357. PA0 Kochba, J., Ben-Hayyim, G., Spiegel-Roy, P., Saad, S. and Neumann, H., 1982. Selection of Stable Salt-Tolerant Callus Cell Lines and Embryos in Citrus sinensis and C. aurantium. Z. Pflanzenphysiol. 106; 111-118. PA0 Nabors, M. W., Kroskey, C. S. and McHugh, D. M., 1982. Green Spots Are Predictors of High Callus Growth Rates and Shoot Formation in Normal and Salt Stressed Tissue Cultures of Oats. Z. Pflanzenphysiol. Bd. 105; 341-349. PA0 Chaleff, R. S., 1983, Isolation of Agronomically Useful Mutants From Plant Cell Cultures, Science, 219; 676-682. PA0 Evans, D. A. and Sharp, W. R., 1983. Single Gene Mutations in Tomato Plants Regenerated From Tissue Culture. Science 221; 949-951. PA0 Rangan, T. S. and Vasil, I. K., 1983. Sodium Chloride Tolerant Embryogenic Cell Lines of Pennisetum americanum (L.) K. Schum. Ann. Bot. 52; 59-64.
It is indicated in the available technical literature that Nabors and co-workers have used tissue culture technology to identify tobacco and oat cells which are tolerant to a single salt (typically NaCl), and have regenerated NaCl tolerant plants from such cultures. Other researchers commonly have failed to report the successful regeneration from their tissue cultures of functioning plants which continue to possess salt tolerance in subsequent generations. Also, the article by Kochba et al points out that tolerance to one salt does not necessarily confer tolerance to other salts. Since saline soils typically contain several different salts in deleterious concentrations, the grower has heretofore received no real aid from tissue culture research to overcome seemingly intractable problems inherent in salt-affected soils. Accordingly, little practical knowledge has been available in the past concerning how to impart meaningful saline tolerance to field grown crops.
It is an object of the present invention to provide plants having an increased tolerance to the presence of a normally deleterious concentration of a plurality of inorganic salts.
It is an object of the present invention to enable growers to successfully grow crops in areas where harmful soil salinity attributable to a plurality of inorganic salts previously made satisfactory yields impossible.
It is an object of the present invention to enable growers of crops to produce enhanced crop yields in saline soils which include a plurality of deleterious inorganic salts.
It is an object of the present invention to provide plants which are capable of both growth and yield in soils containing a normally deleterious concentration of a plurality of water-soluble salts which are comparable to those obtained in a non-saline soil.
It is an object of the present invention to provide an improved tissue culture technique which has been found capable of simultaneously activating a latent genetic mechanism found to be endogenous to a plant which imparts improved tolerance to a plurality of inorganic salts.
It is another object of the present invention to provide an improved process for increasing the tolerance of crops to a plurality of water-soluble inorganic salts which does not require an extensive plant breeding program involving numerous crosses.
It is a further object of the present invention to provide the formation of the first known Linum usitatissimum plants and seed capable of forming the same which exhibit substantial tolerance to a normally deleterious concentration of a plurality of water-soluble inorganic salts.
These and other objects, as well as the scope, nature, and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.